Covertness of radar has persisted as a major problem in defense applications of radiation based sensors. The problem is sometimes known as "the beacon in the sky syndrome." Ordinary airborne radar emits signals from its antenna, and the RF energy travels forward and spreads out in density until the target is intercepted. The target reflects a small amount of the transmitted power back towards the radar. The return energy is compared with that which had been transmitted for target parameters including, among others, distance and rate of change of position, i.e. target speed. Systems such as these are some times referred to as target or range trackers, and the like.
On the other hand, an enemy may employ a search receiver for monitoring the electromagnetic radiation emitted by the radar system. For the radar to work properly, the radar wastes a great amount of energy which is used to an advantage by an enemy search receiver. This is, then, the "beacon in the sky syndrome."
It is well known in the art that to reduce the effectiveness of the search receivers, covert radars should use the widest possible signal spectrum for their transmissions. Additionally, systems may use sequential frequency hopping to spread the energy even further over the electromagnetic radiation spectrum. It is an object of the invention to provide a circuit which provides a wide frequency spectrum signal for radar transmission.
Wide frequency spectrum radar has long been recognized and desired for use in radar systems. However, a wide spectrum high energy system generates a very high energy narrow pulse in the time domain. Unfortunately, radar systems of the prior art are unable to transmit or receive such a high energy narrow pulse. It is therefore an object of the invention to provide a wide frequency spectrum radar signal in such a manner which obviates the need for very high energy narrow pulse transmitter and receiver systems.
It is another object of the invention to provide a covert radar system in which radar signals emitted are comparable to receiver thermal noise and therefore are substantially undetectable in the normal radar operating range.